The functional affinities of antibodies of different IgG subclasses to dietary antigens in mothers and their babies.

The quantity and functional affinities of IgG1 and IgG4 antibodies to the dietary antigens casein and ovalbumin were measured in unselected mothers and their 1-year-old infants. In these infants, the titre of IgG antibodies to both antigens was highest in the IgG1 subclass, while in their mothers the titre of IgG1 and IgG4 antibodies to these foods was similar. High affinity IgG4 responses to both casein and ovalbumin were frequently found in mothers, whilst IgG1 responses, particularly to ovalbumin, were of low functional affinity. By contrast, in the 1-year-old infants, the functional affinity of IgG1 antibody to ovalbumin was substantially higher than in their mothers (Student's paired t-test, P < 0.001), indicating that higher affinity IgG1 antibody was produced on first exposure to ovalbumin rather than following chronic exposure. The effect of antigenic load on affinity maturation was further investigated by comparing the affinity of IgG1 antibody to casein in bottle, mixed and breast fed infants. Bottle fed infants had significantly higher-affinity IgG1 antibodies to casein compared with breast or mixed fed infants (Student's unpaired t-test, P < 0.01 and 0.02), suggesting that antigen exposure via the gut was able to drive the affinity maturation process. In studying the immune response it is clear that account must be taken of the affinity as well as of the titre of the antibody produced.